Ultrasonic transducers are used in a wide variety of applications wherein it is desirable to view the interior of an object non-invasively. For example, in medical applications physicians use ultrasonic transducers to inspect the interior of a patient's body without making incisions or breaks in the patient's skin, thereby providing health and safety benefits to the patient. Accordingly, ultrasonic imaging equipment, including ultrasonic probes and associated image processing equipment, has found widespread medical use.
Ultrasonic probes provide a convenient and accurate way of gathering information about various structures of interest within a body being analyzed. In operation, ultrasonic probes generate a signal of acoustic waves that is acoustically coupled from the probe into the medium of the body so that the acoustic signal is transmitted into the body. As the acoustic signal propagates through the body, part of the signal is reflected by the various structures within the body and then received by the ultrasonic probe. By analyzing a relative temporal delay and intensity of the reflected acoustic waves received by the probe, a spaced relation of the various structures within the body and qualities related to acoustic impedance of the structures can be extrapolated from the reflected signal.
In operation, previously known medical probe generate a signal of acoustic waves using a plurality of piezoelectric elements. Despite the plurality of the piezoelectric elements, the elements are arranged proximate to one another so that the probe effectively has a single acoustic aperture integral with a top portion of the probe. The signal is acoustically coupled from the effective acoustic aperture of the probe into the medium of the patient's body, so that the signal is transmitted into the patient's body. Typically, this acoustic coupling is achieved by pressing the top portion of the probe into contact with a surface of the abdomen of the patient.
As the weakly reflected acoustic waves received by the probe propagate there through, they are electrically sensed by electrodes coupled to the probe. A large number of small probe electrodes are preferred to provide high resolution and control of a small, easy to handle, probe. Unfortunately, there are some difficulties in manufacturing the large number of small probe electrodes and in providing electrical coupling to the electrodes, because of the small size and complexity.
By analyzing a relative temporal delay and intensity of the weakly reflected waves received by the medical probe, imaging system components that are electrically coupled to the electrodes extrapolate an image from the weakly reflected waves to illustrate spaced relation of the various tissue structures within the patient's body.
Since the human body is not acoustically homogeneous, different frequencies of operation of an ultrasonic probe are desirable, depending upon which structures of the human body are serving as an acoustic transmission medium and which structures are the target to be imaged. Many commercially available ultrasonic probes include a transducer array that is optimized for use at only one particular acoustic frequency. Accordingly, when differing applications require the use of different ultrasonic frequencies, a user typically selects a probe which operates at or near a desired frequency from a collection of different probes. Complexity and cost of the ultrasonic imaging equipment is increased because a variety of probes, each having a different operating frequency, is needed. An economical and reliable alternative to manually coupling different transducers to such imaging systems is needed.
Previously known dual frequency ultrasonic probes utilize a transducer with a relatively broad resonance peak. Desired frequencies are selected by filtering. Current commercially available dual frequency probes typically have limited bandwidth ratios, such as 2.0/2.5 MHz or 2.7/3.5 MHz. Graded frequency ultrasonic sensors that compensate for frequency downshifting in the body are disclosed in U.S. Pat. No. 5,025,790, issued Jun. 25, 1991 to Dias. Dual frequency ultrasonic probes can additionally provide for added flexibility in "color flow" mapping wherein a first frequency is used for conventional echo-amplitude imaging and a second frequency is used for doppler shifted flow imaging.
While such previously known dual or graded frequency ultrasonic probes provide some advantages, variable control over size of the effective acoustic aperture of the probe is also needed. To maintain good image quality, it is desirable to maintain size of the effective acoustic aperture of the probe at a constant number of wavelengths of the acoustic signal. Accordingly, to maintain good and uniform image quality as frequency and therefore wavelength of the acoustic signal is varied, it is desirable to vary size of the acoustic aperture so that the size corresponds to a constant number of wavelengths of the signal. In the field of underwater sound transmitting or receiving systems used by the U.S. Navy at frequencies ranging from fifty to two hundred and fifty kilohertz, a stainless steel acoustic filter plate is used to provide an effective acoustic aperture diameter that is a constant multiple of the acoustic wavelength of the sound in the underwater medium, as explained in U.S. Pat. No. 4,480,324 issued to Sternberg. Because this patent provides helpful background information, it is incorporated herein by reference.
While the stainless steel filter plate provides some advantages, it has limited use in medical imaging applications because of its size, weight, and complexity and because medical imaging applications require operation at frequencies much higher than two hundred and fifty kilohertz operation of the plate. Since there is little equipment space available in hospital facilities, it is particularly important that the probe be compact.
What is needed is a tunable ultrasonic probe that provides efficient electrical coupling to imaging system components, while further providing variable control over size of the effective acoustic aperture of the probe.